Minecraft Beta Program Question: Old Worlds

I am guessing your friend is enrolled into the beta for Minecraft through the xbox insider program

Actually, they sent the beta out to non beta users, and as noted above if they tried to load in their old world, it may be (temporarily) lost. As of 30 minutes ago (via the xbox feed) they advised those who got the beta to uninstall it and reinstall Minecraft
on their xbox.

Your son is playing on the official release channel for Minecraft. His 1.12 version is currently the latest official version, whereas his friends have joined the Insider Program and are playing a beta test version of Minecraft.

His friends can either un-enroll from the Beta Insider Program, or you son can try to join the Insider Program and update his Minecraft to a beta version.

Be aware however, that beta version still contain untested parts, and the game can crash, malfunction and in turn destroy worlds if an error occurs.

If your son is still willing to test the Beta, he can download the free Insider app from the store, and join the Minecraft Beta.

Then he'll have to download/update his Minecraft to the newer beta version.

As Ackis already said: since this is an old thread, please create your own new thread.

In the mean time, ask the friend you're trying to connect to, if they joined the Minecraft Beta Insider program. if they are using a beta version that is newer than your regular version, you'll get that error.

Either ask your friend to leave the Insider Program for Minecraft, or join the Minecraft beta program yourself by downloading the Xbox Insider App from the Marketplace.

Is it possible that one of you or your friends has joined the Minecraft Beta Insider program?

If so, you probably have different versions of the game, and only players with the same Minecraft version can join each other's worlds.

Check that you all have either the official Minecraft version of 1.11.4. If one (or more) have the current Beta version of 1.13.something, then you can't join each other's worlds.

If one of you have the beta version of Minecraft available through the Xbox Insider Program/App on the Xbox One it will not let you join each other's worlds unless both of you have that version or the person who does have it goes through said program and
de-enrolls the game from it. Try looking that up if what was suggested by the mod doesn't work. The Insider Minecraft beta is free and it shouldn't delete your world saves if you use it or happened to have had it installed.